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Electric cars, which have soundless engines, would need to make noises to let pedestrians know they're near, under a US proposed rule released recently.

Sounds would need to be detectable when vehicles are travelling slower than 18 miles per hour (29 kilometres) so electric and hybrid-electric cars can be heard by bicyclists and pedestrians, particularly the visually impaired, under the US National Highway Traffic Safety Administration (NHTSA) rule.

The so-called quiet-car rule, which would have to be made final before it takes effect, would save 35 lives over each model year of hybrid vehicles and prevent 2,800 injuries, the agency said in an e-mailed statement.

"To add about a $30 or $35 item to a car for this kind of injury and death prevention, it's hard to argue against," Jesse Toprak, an analyst for industry data provider TrueCar.com in Santa Monica, California, said in an interview. "I'm sure all of us have experienced at some time the fear of getting struck by a Prius."

Adding external speakers to quiet vehicles would cost about $25 million a year, or about $35 per light vehicle, NHTSA said. About $1.48 million of the annual costs would be to equip large trucks and buses and motorcycles with sound, the agency said.

Toyota Motor equips its hybrid cars sold in Japan, including the Prius, to emit a sound when the engine is run, even though there are no such regulations in the country, Shino Yamada, a Tokyo-based spokeswoman for Asia's largest automaker, said by telephone today.

No noise"Our proposal would allow manufacturers the flexibility to design different sounds for different makes and models while still providing an opportunity for pedestrians, bicyclists and the visually impaired to detect and recognise a vehicle and make a decision about whether it is safe to cross the street," NHTSA Administrator David Strickland said in the statement.

That cost compares with $2.7 billion a year for a rule NHTSA didn't issue by its deadline last week to require backup cameras in all new cars. Both rules were required by laws passed by the US Congress.

The quiet-car rule's cost assumes hybrid vehicles are 4.1 per cent of US light-vehicle sales. Last year, hybrids were 3 percent of light vehicles sold in the US, up from 2.1 per cent in 2011, said Alan Baum, principal of industry researcher Baum & Associates in West Bloomfield, Michigan. Plug-in electric vehicles were 0.37 per cent of sales last year, up from 0.14 per cent the previous year, Baum said.

The National Federation of the Blind had pushed for the rule, saying an increasing number of cars that don't make noise at low speeds put blind people at risk.

'Pandora's box'"We understand that the proposal allows for some flexibilities in the specifics of the simulated sound autos will produce, while avoiding opening a Pandora's box of sounds," Wade Newton, a spokesman for the Alliance of Automobile Manufacturers, said in an e-mail. "The Alliance will continue working with National Federation of the Blind and others, to work toward this being the model for an international safety standard." Above 18mph, cars make enough noise that they don't need artificial sound, NHTSA said.

Hybrid-electric cars such as General Motors Co.'s Chevrolet Volt use electric engines at low speeds and can switch to internal-combustion engines. Fully electric cars such as the Nissan Motor Co. Leaf use only battery power.Cars that aren't using internal-combustion engines don't make noise while they accelerate or idle like vehicles that are powered by gasoline or diesel fuel. The Pedestrian Safety Enhancement Act of 2010 requires the US Transportation Department to write a rule addressing this issue by January 4, 2014.

Quiet cars are twice as likely as vehicles with internal-combustion engines to be involved in pedestrian accidents when backing up, starting in traffic or entering or leaving a parking space or driveway.